Temperature compensation oscillators (TCXOs) used in various fields are in heavy use in portable mobile communication devices such as a cellular phone and so on in recent years. Generally, as this kind of temperature compensation oscillator, a crystal oscillator is widely used in which an oscillation circuit is constituted of a 10 MHz band AT cut quartz crystal (resonator) as an oscillation source and provided with a temperature compensation circuit so that the temperature characteristic in a cubic curve of the AT cut quartz crystal is cancelled to stabilize the oscillating frequency.
For these kinds of temperature compensation oscillators, a reduction in size and weight and a reduction in price as well as stability of an oscillation output signal are desired. For these needs, several types of packages are known. For example, the packages include a single type in which a quartz crystal (piezoelectric element) being a resonator and an integrated circuit constituting the temperature compensation circuit are mounted in the same chamber in the package, a double type in which a quartz crystal and an integrated circuit are separately packaged and bonded together, and an H type in which a quartz crystal and an integrated circuit are mounted in front and back separate chambers with a middle partition held therebetween.
A package configuration example of a single-type surface-mounted temperature compensation oscillator is now shown in FIG. 14.
This temperature compensation oscillator has a package (container) 10 which is constituted of a package main body 11, a welded ring 12, and a cover 13, to the inside of which a quartz crystal 15, and a MOS IC (integrated circuit) chip 16 constituting an oscillation circuit and a temperature compensation circuit which will be later described are attached and sealed in the same chamber. Note that, in addition to the IC chip 16, a circuit element such as a chip capacitor or the like may also be mounted in the package main body 11.
The temperature compensation oscillator has a circuit configuration as shown in FIG. 15. An oscillation circuit 20 forms an inverter oscillation circuit in which the quartz crystal 15 being a piezoelectric element, an inverter 21, and a feedback resistor 22 are connected in parallel, and their both connection points are grounded via DC cut capacitors Cc and Cd and voltage variable capacitors (voltage-controlled variable capacitance condensers) 23 and 24 which are oscillation capacitors, respectively. An oscillation output signal is outputted from the connection point on the output side of the inverter 21 to an output terminal 26.
Furthermore, a temperature detection circuit 18 which detects the temperature state near the quartz crystal 15 in the oscillation circuit 20 and a temperature compensation circuit 30 which controls to keep the oscillation frequency of the oscillation circuit 20 substantially constant based on a temperature detection signal from the temperature detection circuit 18, are provided.
The temperature compensation circuit 30 comprises a compensation data storage circuit (non-volatile memory) 31 which stores compensation data and a D/A conversion circuit 32 which generates a voltage signal as a temperature compensation signal based on the compensation data and the temperature detection signal from the temperature detection circuit 18. Then, the voltage signal is applied to terminals on the non-grounded side of the voltage variable capacitors 23 and 24 via resistors R1 and R2 provided in the oscillation circuit 20 respectively, so as to change the capacitances of the voltage variable capacitors 23 and 24 in accordance with the voltage, thereby controlling the oscillation frequency of the oscillation circuit 20 to keep the frequency of the oscillation output signal substantially constant.
In such temperature compensation oscillators, all of the quartz crystals 15 and the oscillation circuits 20 formed in the IC chips 16 cannot be formed completely the same due to variation in manufacturing or the like, and therefore they will individually have different temperature-frequency characteristics. Accordingly, all of the oscillation circuits 20 cannot be temperature-compensated based on the same reference. Therefore, it is necessary to create individual compensation data different for each oscillation circuit and store it into the compensation data storage circuit 31. However, the oscillation circuits cannot be sufficiently compensated if the quartz crystals 15 exhibit a wide range of characteristic variation, and it is therefore necessary to adjust as much as possible the characteristics of the quartz crystals 15 in advance.
Therefore, there is an adjustment method in which the IC chip constituting the oscillation circuit is not mounted when the characteristics of the piezoelectric element such as the quartz crystal and the like are adjusted, but the piezoelectric element is caused to resonate and its resonant frequency is monitored from the outside by the network analyzer or the like and the electrode film on the surface of the piezoelectric element is removed or added so that the frequency has a desired value.
However, this adjustment method has suffered from a problem that deviation arises between the oscillation frequency when the IC chip is also mounted in the package and the circuit is caused to perform oscillation operation and the previously adjusted resonant frequency. In addition, the number of adjustment steps has been large which requires extra adjustment cost.
To solve such problems, a temperature compensation oscillator is proposed which is configured such that, in a state in which a piezoelectric element such as a quartz crystal and an IC chip are mounted in a package to constitute a temperature compensation oscillator, its oscillation circuit is operated to allow the temperature characteristic of the piezoelectric element itself to be accurately adjusted and the work to create the temperature compensation data and store it into the temperature compensation data storage circuit can also be subsequently appropriately performed, thereby simplifying and increasing the accuracy of the adjustment process (see, for example, Patent Document 1).
The temperature compensation oscillator is provided with a selection device selecting whether to bring the temperature compensated function of the temperature compensation circuit into an enabled state or a disabled state, and is caused to operate as a simple oscillator by bringing the temperature compensation function into the disabled state when adjusting the electrode film on the piezoelectric element so that the oscillation frequency is a desired frequency at a reference temperature (room temperature).
Concretely, a selection circuit using a constant voltage generation circuit and two pairs of transmission gates is provided in addition to the temperature compensation circuit, and the transmission gates of the selection circuit are switched over to apply a temperature compensation signal (voltage signal) from the temperature compensation circuit to a voltage variable capacitor of the oscillation circuit to thereby control its capacitance according to the temperature when bringing the temperature compensation function into the enabled state, and to apply a constant voltage from the constant voltage generation circuit to the voltage variable capacitor to fix the capacitance to a predetermined value when bringing the temperature compensation function into the disabled state.
Patent Document 1: JP 2003-218636A (page 4-9, FIG. 1)